This invention relates generally to telephone subscriber switching systems and more particularly to a subscriber system in which subscriber traffic is concentrated at a remote line switch portion of a central office switching system and passed between remote line switch portion and central office switching portion in concentrated form. The remote line switch is a concentrating portion of the central office switching network and is controlled by existing central office equipment over multichannel data links which forward status and control signals between the remote line switch and the central office.
In conventional telephone practice each subscriber telephone instrument is connected to the telephone central office by an individual pair of wires called the subscriber line (or loop). Particularly where long distances are involved, the cost of providing individual lines between a group of subscribers and the serving central office is high. Various means have been used to provide more economic alternatives. These means include subscriber carrier systems, subscriber concentrator systems, and combinations thereof.
A multichannel subscriber carrier system uses frequency division or time division multiplexing to provide many communication channels over one or two pairs of wire. Subscriber lines are individually connected to a remote carrier terminal, their corresponding central office "appearances" are connected to a central office carrier terminal, and the carrier system provides dedicated transmission channels between the two carrier terminals.
A subscriber concentrator system takes advantage of the fact that only a small fraction of a group of subscribers will require telephone service at a given time. Subscriber lines are individually connected to a remote concentrator terminal, their corresponding central office appearances are connected to a central office concentrator terminal, and the two terminals are connected to one another by pairs of wire or transmission channels called concentrator trunks. Because of the random statistics of subscriber service requirements, the number of concentrator trunks can be substantially less than the number of subscribers served by the concentrator system. Typical concentration ratios are of the order of four to one.
Each concentrator terminal contains a switch to selectively connect subscriber lines and corresponding central office line appearances to the concentrator trunks. The two switches must be operated cooperatively, such that when the central office concentrator switch connects a particular subscriber line appearance to a particular concentrator trunk the remote concentrator switch will connect the same concentrator trunk to the corresponding remote subscriber line. Therefore, the concentrator system requires a self-contained control system to control the operation of its two switches, and to recognize and respond to requests for subscriber connections (and disconnections) which may be initiated from either the remote terminal or the central office terminal. The concentrator control system requires additional transmission paths between the concentrator terminals.
A telephone central office switching machine also takes advantage of the fact that only a small fraction of the subscribers will require telephone service at a given time. When a subscriber concentrator system is associated with a central office switching machine, the combination includes an unnecessary duplication of function.